For determining mass flow rate of a fluid flowing in a pipe, especially that of a liquid, Coriolis mass flow meters are often used. As is known, these meters cause Coriolis forces in the fluid by means of a corresponding vibration-type sensor, driven by a control—and evaluation-electronics connected thereto, and use these forces to produce a measurement signal representing the mass flow rate.
Coriolis mass flow meters are long known and are used industrially. Examples of such meters are presented in EP-A 1 154 243, U.S. Pat. Nos. 4,876,898, 4,801,897, 5,048,350, 5,301,557, 5,394,758, 5,549,009, 5,796,011, and WO-A 02/099363, which describe Coriolis mass flow meters, each having a sensor comprising:                a double tube arrangement having                    a first sensing tube vibrating in operation and            a second sensing tube vibrating in operation,            wherein the vibrating sensing tubes oscillate essentially oppositely in phase to one another, and            wherein at least one of the two sensing tubes is in communication with the pipe,                        an oscillations exciter for driving the sensing tubes, as well as        oscillation sensors for registering inlet and outlet oscillations of the sensing tubes and for producing at least one mass-flow-influenced, electrical, sensor signal,        wherein the oscillations exciter and/or the oscillation sensors have at least one magnetic circuit arrangement for transforming electrical energy into mechanical energy and/or vice versa,        wherein the at least one magnetic circuit arrangement comprises                    at least one coil attached to the first, vibrating, sensing tube of the sensor, at least at times carrying an electrical current, and at least at times traversed by a magnetic field, and connected by means of at least one pair of electrical leads with a measuring device electronics of the Coriolis mass flow meter, as well as            an armature attached to the second, vibrating, sensing tube of the sensor and interacting with the at least one coil.                        
The two sensing tubes are, as usual for such sensors, connected mechanically together by means of an inlet distributer piece and an outlet distributor piece, and/or by means of an inlet node plate and an outlet node plate.
Curved or straight sensor tubes of such sensors can, as is known, when excited in the so-called “useful mode” to bending oscillations of a first natural oscillation form, cause Coriolis forces in the fluid flowing therethrough. These, in turn, lead to a situation where bending oscillations of a so-called “Coriolis mode” are superimposed on the excited bending oscillations of the useful mode, and, because of this, the oscillations registered by means of the inlet and outlet oscillation sensors exhibit a measurable phase difference dependent on the mass flow rate.
Usually, the sensor tubes are excited in operation to an instantaneous resonance frequency, especially in the case where oscillation amplitude is regulated to be constant, such that they oscillate essentially perpendicularly to a longitudinal gravity line of the double-tube arrangement established by the two tubes. Since the resonance frequencies of the sensor tubes depend, among other things, also on the instantaneous density of the fluid, conventional Coriolis mass flow meters also permit the density of the flowing fluid to be measured, in addition to the mass flow rate.
In the case of magnetic circuit arrangements for the above-mentioned sensors, both the armature and the associated coil are always attached directly to the double-tube arrangement, so that, in operation, both follow the movements of the associated sensor tubes. This, in turn, means that the coil is repeatedly moving relative to a neighboring attachment location, where the electrical leads of the coil are intercepted on the sensor.
For preventing mechanical overloading of these leads, e.g. due to repeatedly arising or permanently alternating bending loads, special measures have already been realized, or discussed in the trade, for arranging the leads in conventional sensors.
Thus, to minimize the amplitudes of oscillatory movements of such electrical leads, they are usually held directly on the sensing tube, where the coil is fixed, and led along the same until reaching a clamping location, where the vibrating sensing tube is constrained such that it does not move during operation. In the case of double-tube arrangements having two sensing tubes traversed by fluid, the interception of the electrical supply lead can occur e.g. in the vicinity of the mentioned distributor pieces, which, as is known, divide the fluid flow into the two connected sensing tubes and then subsequently reunite such. Apart from the fact that, by placing the leads directly on the sensing tube, the tube oscillation properties can be influenced, such placement is not even possible, when the sensor is to be used for measuring high temperature fluids.
A further remedial possibility applicable also in the case of this problem, especially in high temperature applications, for preventing mechanical overloading of the electrical supply lead, especially in the case of sensors suited for fluids of high temperature, is e.g. described in U.S. Pat. No. 4,738,143 or 4,876,898. For the sensors disclosed there, which can serve, for example, even at temperature ranges above 400° C., the oscillatory movements of the electrical leads are relieved within a relatively short lead section by means of an elastically bending and electrically conducting spring element, which is inserted into the lead. Each spring element is coupled for this purpose at a first end both mechanically and electrically to the coil and at a second end non-conductively to a housing containing the sensor or to a supporting frame attached to the housing. Then, from the second end of the spring element serving here as an attachment point for the electrical supply lead, the supply lead, now practically fully uncoupled from the oscillations of the coil, is extended further in suitable manner, for example as a conductive rail and/or insulated wire.
Due to the operationally imposed, constant bending loading at mostly high temperatures, the spring elements must have a relatively high, largely temperature-independent elasticity, coupled with a very high resistance to aging. Nickel alloys are proposed as material for the spring elements. On the one hand, the spring elements must, namely, be stable over a long time period. On the other hand, however, the spring elements cannot be too difficult to bend, since, otherwise, an influencing of the bending oscillations of the sensing tubes, and, thus also, the sensitivity of the sensor to the parameter to be registered, becomes possible to an undesirably high extent. In order to be able to satisfy all these requirements together, at least approximately, especially also to attain the required reliability, the spring elements become comparatively expensive to manufacture. Then, there is the necessity for mounting them at very great technical effort onto the sensing tubes and, as mentioned above, onto the sensor housing, or the like.
Another possibility for avoiding overloading of the electrical supply leads is proposed, for example, in U.S. Pat. No. 5,349,872 and WO-A 02/088641. In the case of the sensors disclosed there, the coils provided for the magnetic circuit arrangement are secured by means of an elastically bendable mount attached to the sensing tubes of the particular double-tube arrangement in such a way that the coils stay in their static rest position in spite of the vibrating sensing tubes. Because of this, the electrical supply leads, too, only experience a negligibly small amount of oscillation-related bending loading during operation of the sensor. However, it has been found in the case of this kind of magnetic circuit arrangement that especially the sensor signals produced therewith can be considerably corrupted, a fact which can, on the one hand, be related to the way in which the magnetic field is channeled, but, on the other hand, can also be traced back to parasitic vibrations of the magnetic circuit arrangement itself.